Fix early-bird price blending and mislabeling; add contact-only events

- Early-bird pricing: RegistrationOption now tracks each purchase as a
  separate price tranche instead of overwriting a single price/quantity
  on repeat purchases, so buying more tickets after a tier expires no
  longer re-prices tickets already bought at the old price. Stock-limit
  checks, total-due calculation, and the Finance report's revenue-by-
  option are all tranche-aware; pages that showed one blended price per
  line now render/total each tranche. Viewing a pending/partially-paid
  registration (dashboard, detail page, or an event's registration
  list) now refreshes stale pricing on the spot instead of only at
  payment time.
- Fixed the "(early bird)" dashboard label incorrectly firing on any
  line priced below the base option price (e.g. a plain cheaper
  variant) — it now checks the real applied-tier flag.
- Added contact-only events (e.g. baptism): no registration/payment
  flow, shown on the public site with a "Contact us" popup instead of
  a Register button. Configurable via the admin event wizard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/prisma/migrations/20260820103708_init/migration.sql

@@ -0,0 +1,16 @@
+/*
+  Warnings:
+
+  - You are about to drop the column `feeAmount` on the `EventCashupLine` table. All the data in the column will be lost.
+  - You are about to drop the column `feeAmount` on the `Payment` table. All the data in the column will be lost.
+  - You are about to drop the column `feeChannel` on the `Payment` table. All the data in the column will be lost.
+  - You are about to drop the column `feeRate` on the `Payment` table. All the data in the column will be lost.
+
+*/
+-- AlterTable
+ALTER TABLE "EventCashupLine" DROP COLUMN "feeAmount";
+
+-- AlterTable
+ALTER TABLE "Payment" DROP COLUMN "feeAmount",
+DROP COLUMN "feeChannel",
+DROP COLUMN "feeRate";